The electric motor is present, for example, as a direct-current electric motor that can be connected via the switching device to the current source, embodied as a direct-current source, by corresponding adjustment of the switching device. The current source or direct-current source may have a constant voltage. If the electric motor is then continuously connected to the current source, a specific rotation speed of the electric motor, dependent on the voltage of the current source, is consequently established.
In order to adjust the rotation speed, therefore in particular to control it in open- and/or closed-loop fashion, provision is made to adjust, with the aid of the switching device, the effective operating voltage that is present at the electric motor. A pulse width modulation is performed, for example, for this purpose, in which for multiple successive working cycles, within each individual one of the working cycles a specific activation ratio is identified and is established at the switching device. The electric motor therefore periodically may be connectable to the current source by way of the switching device.
The activation ratio identifies the ratio between the duration of an activation time period—the only one in which the switching device is rendered conductive—and the total duration of the corresponding working cycle. If the effective operating voltage is to correspond to the voltage furnished by the current source, the switching device must then be rendered conductive for the entire working cycle, so that the duration of the activation time period corresponds to the duration of the respective working cycle.
In the at least one operating mode, however, the effective operating voltage is intended to be lower than the voltage furnished by the current course. The switching device is accordingly not rendered conductive over the entire working cycle, so that the working cycle is made up of the activation time period and the freewheeling time period; during the latter, the switching device is not switched, i.e. the electrical connection between the current source and the electric motor is interrupted. The desired effective operating voltage, and consequently the desired rotation speed of the electric motor, can be adjusted in this fashion, in particular adjusted in open- and/or closed-loop fashion.
Because the electric motor or its motor windings have a high inductance, a current dissipation path is necessary after each activation time period. This path is furnished via the freewheeling device, which is connected in parallel with the electric motor. The freewheeling device can be, for example, a diode, which in accordance with its function can also be referred to as a “freewheeling diode” and is reverse-biased with respect to the electric motor. This means that during the activation time period no electrical current flows through the freewheeling device, so that the current is completely available to operate the electric motor. In the freewheeling time period, conversely, the motor current flowing through the electric motor can be dissipated via the freewheeling device.
The embodiment of the freewheeling device as a freewheeling diode has the advantage that no active control application at all, for example by way of a control unit, is necessary: the diode blocks automatically as soon as the motor current is dissipated. The freewheeling diode thus represents a passive freewheeling device. The disadvantage of this circuit, however, is a relatively high power loss, which is determined by the diode's forward voltage and the motor current. This is particularly disadvantageous with electric motors that are operated with high current and a low operating voltage, which is often the case in the automotive sector.